Lensmeters for the measurement of eyeglass prescription are well known. This invention relates to lensmeters. More particularly, this invention relates to lensmeters for measuring contact lenses without the adverse effects of spherical aberration inherent in such measurements. Of late, such lensmeters have been automated. Such automated lensmeters have the advantage of not requiring operator interaction with the measuring apparatus. The operator's role with such machines is purely passive--all the operator does is to insert the lens to be measured in the meter and thereafter receive a reading--verifying or determining the power of the lens in sphere, cylinder, and axis.
Contact lenses and their measurement have posed a problem for lensmeters, whether the lensmeters are manual or automatic. Unfortunately, the users of manual automatic lensmeters fail to understand why such lensmeters cannot measure contact lenses accurately. There is a tendency to fault the manufacturer of lensmeters when the user realizes contact lenses cannot be accurately measured.
The problem in measuring contact lenses originates in the highly meniscus form of such lenses. This problem may be best understood by considering the optical effect of a contact lens on the human eye--and thereafter understanding the optical effect of such lenses when they are removed from the eye for measurement of their optical properties.
A contact lens maintains its position on the human eye by forming an interface of fluid contact between the posterior surface of the contact lens and the eye at the cornea. Simply stated, the ambient tear layer of the eye causes the contact lens to stay positioned on the eye.
The power of the contact lens is determined by the difference in optical power between the posterior and anterior surfaces of the contact lens. When the contact lens is on the eye, the posterior surface-to-eye interface has very little refractive effect on light entering the eye and the highly meniscus form of the lens conforms to the natural optics--also highly curved--of the eye. The difference in optical power between the posterior and anterior surfaces of the contact lenses imparts to the cornea changed prescription. There results corrected vision to the wearer of contact lenses.
In order for the contact lens to stay on the average human eye, considerable lens curvature is required. This lens curvature is in the order of 8 millimeters (mm) radius of curvature. While the effect of this curvature is largely not present when the lens is worn on the eye, the effect of the curvature becomes pronounced when the contact lens is removed from the human eye for testing This effect is present largely in the form of spherical aberration.
Spherical aberration is known. This effect can be simply understood considering the case of a highly curved refracting optical surface. It is found that bundles of rays entering the surface all parallel to the optical axis do not come to a common point of focus after being refracted by that surface. Rays which enter the surface at a common distance from the optical axis, or zone, do come to a common focal point, the zonal focus for that distance The distance of that zonal focal point from the surface is called the zonal focal length and it is different for each zone. The resulting "focus" of such refracting surfaces is said to exhibit "spherical aberration." Stated in other terms, the focus of the suspect optics having spherical aberration is distorted.
Unfortunately, lensmeters have heretofore relied on the basic assumption that the focal power of the suspect optics is constant over that suspect optic or at least constant in a given meridian in the case of an astigmatic optic. If the lensmeter is of the type which takes its sample over an extended aperture, it is found, in the case of spherical aberration, that since there is no one focal length for all zones of the aperture, one single, well-defined focal length cannot be found. If the lensmeter is of a type which restricts its sample to a single zone, an unique focal length can be found in the presence of spherical aberration but this focal length is different depending on which zone is picked
Taking the case of a contact lens, with successive positive (anterior) and negative (posterior) refracting surfaces in the range of 60 diopters, significant spherical aberration will be found when the effective power of the lens becomes great. It is typically desired to find the paraxial power of a contact lens because this most closely gives its effect when worn on the eye. Unfortunately a lensmeter measuring zonal focal measuring zonal focal length cannot directly measure in the infinitely small zone corresponding to a paraxial power but must measure in larger zone and so will find a power different from the paraxial power. In the case of lensmeter measuring over a full aperture of many zones it is very difficult to decide on the desired paraxial focus because there is no clear-cut focal point. Unfortunately, users of lensmeters, especially automated lensmeters, ascribe the failure of such measurements in the case of contact lenses to the particular lensmeter and its manufacturer.